1. Field of the Invention
The invention relates to a method of producing fertilizer grade DAP having increased nitrogen concentration. Fertilizer grade DAP is defined in Official Publication No. 52 (dated 1999) of the Association of American Plant Food Control Officials as a product composed of ammonium phosphates, principally diammonium phosphate, resulting from the ammoniation of phosphoric acid.
2. Background of the Invention
Phosphorus as well as nitrogen are essential plant nutrients. Phosphorus and nitrogen ensure cell multiplication and thus growth since both are structural components of nucleic acids. Plants use the phosphorus and nitrogen to grow. Since fertilizers are generally formed by the reaction of phosphoric acid and ammonia, they are used to provide a rich source of nitrogen and phosphorus to the plants. Such fertilizers are generally applied to the soil and are readily assimilable by plants.
The phosphoric acid used in fertilizers is usually manufactured from rock phosphate typically in one of two ways. Phosphate rock is composed chiefly of calcium phosphates and calcite.
In the first prior art method, commonly known as the furnace acid process, the phosphoric acid is made by heating phosphate rock to a form of relatively pure elemental phosphorus which is then converted into the phosphoric acid. More particularly, this prior art process uses an electric furnace heated to about 1500.degree. K. The phosphate rock is reduced by coke in the presence of sand or silicon dioxide according to the following reaction: EQU Ca.sub.3 (PO.sub.4).sub.2 +3SiO.sub.2 +5C=3CaSiO.sub.3 +5CO+P.sub.2
Gaseous P.sub.2 condenses to form a solid P.sub.4. In the air, the P.sub.4 converts to phosphorus pentoxide. Water is then added to the phosphorus pentoxide to form the phosphoric acid. The process usually yields 85 percent phosphoric acid which is colorless and has a density of about 1.69 kg/L. The phosphoric acid formed by this prior art process generally yields a purer phosphoric acid as compared to the phosphoric acid formed by the second prior art process, which is described below. However, this first process is also much more costly than the second prior art process.
The second prior art method used to produce phosphoric acid is known as the wet process. Most of the phosphoric acid produced in the United States is produced by this process. In this wet process, finely ground phosphate rock is slurried with sulfuric acid. Sometimes, recycled dilute phosphoric acid is included in the slurry. The reaction that occurs in this wet process is set forth below: EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3 PO.sub.4
In addition to the orthophosphoric acid, the reaction yields gypsum and numerous other suspended and dissolved impurities. The slurry is generally filtered to remove the solid impurities, mainly, gypsum. The resultant filtrate contains between 25 and 35 percent P.sub.2 O.sub.5 and between 1 and 8 percent by weight of suspended solids or impurities that are not removed by the filtration process. Examples of wet process methods are disclosed in U.S. Pat. Nos. 4,487,750, 4,485,078, 4,657,559, 4,665,790 and 4,655,789, which are incorporated herein in their entirety. U.S. Pat. No. 4,710,366 discloses methods of removing further impurities from the filtrate; such patent is incorporated herein in its entirety.
As used herein, the terms phosphoric acid and wet process phosphoric acid mean orthophosphoric acid.
Phosphoric acid is reacted with anhydrous ammonia to form ammonium phosphates, which constitute a large class of phosphorus fertilizers. See Examples 7 and 9 of U.S. Pat. No. 4,485,078. The common ammonium phosphates include monoammonium phosphate and diammonium phosphate, which are the primary components of the fertilizers commonly known as MAP and DAP, respectively. The phosphoric acid is preferably reacted with the proper proportion of anhydrous ammonia which primarily provides diammonium phosphate, which is a source of nitrogen and phosphorus readily assimilable by food crops.
An example of the formation of ammonium phosphate from wet process phosphoric acid is described in Manual of Fertilizer Processing, edited by Francis T. Nielsson, Marcel Dekker, Inc., (1987), on page 253 under the title "Spray Tower Process". The article notes that in 1965, L. B. Nilsson described a spray tower process for producing ammonium phosphate from wet process phosphoric acid. The acid was sprayed into an ammonia atmosphere in a tower. The heat of reaction between the ammonia and the acid was sufficient to evaporate the water in the acid droplets, and the product was removed as a dry powder from the bottom of the tower. To make DAP, phosphoric acid with a minimum concentration of about 45 percent P.sub.2 O.sub.5 was required. A weaker acid solution might contain more water than what could be evaporated by the heat of reaction. If the heat of reaction was not sufficient to evaporate the water in the acid and compensate for any losses from the tower, additional heat had to be supplied. The product from the process was not as acceptable because the material was a fine powder and the bulk density was low. A granulation step would be required for product acceptance. The only compound mentioned in the article is diammonium phosphate, which implies an N/P mole ratio of 2. There is no mention in the article of triammonium phosphate (N/P mole ratio of 3), or production thereof. Also, there is no mention in the article of the use of elevated pressures.
The TVA (Tennessee Valley Authority) in the early 1960s developed a process for the preparation of granular DAP from ammonia and phosphoric acid. The conventional TVA process for preparing granular DAP is described on pages 248 to 251 of Manual of Fertilizer Processing, ibid. The conventional TVA type of process has a preneutralizer for partial ammoniation of the phosphoric acid and completion of ammoniation is done in a rotary ammoniator-granulator. Granulation is controlled by recycling product fines to the drum. The basic TVA process involves partial preneutralization of the acid in a preneutralizer (reaction tank) followed by completion of ammoniation to DAP in the rotary ammoniator-granulator. Excess ammonia, which must be fed to the ammoniator-granulator to produce DAP, is recovered by scrubbing the off gases with the acid to be used in the process. The granular product is normally dried, cooled, and screened, having the undersized and crushed oversized granular DAP recycled to the granulator to control granulation.
Pure diammonium phosphate [(NH.sub.4).sub.2 HPO.sub.4 ] is also termed dibasic ammonium phosphate. Page 561 of The Merck Index, 10.sup.th Ed., (1983), states that diammonium phosphate gradually loses about 8 percent NH.sub.3 on exposure to air.
Triammonium phosphate is (NH.sub.4).sub.3 PO.sub.4.
DAP (fertilizer grade) is defined as a product composed of ammonium phosphates, principally diammonium phosphate, resulting from the ammoniation of phosphoric acid.